Manufacturers and users of these tubes are constantly concerned with improving the mechanical behaviour of tubes manufactured by the extrusion of plastics material, the main characteristic feature of such tubes being their resistance to bursting and their behaviour to flow with internal pressure.
As a first approximation, the stresses produced in a tube of low thickness in relation to the radius, with an internal pressure P are given by the equations: ##EQU1## with .sigma..sub.L : stress in the longitudinal extent
.sigma..sub.R : stress in the radial extent PA1 e: thickness of tube wall PA1 R: average radius of tube
The pull is approximately twice as great in the radial extent as in the longitudinal extent.
For a homogeneous, isotropic substance (that is to say one where the mechanical properties are independent of the direction of pull) in the form of a tube subjected to an internal pressure P, the tube can rupture by swelling and bursting when a crack develops in the longitudinal extent.
Simple methods for extruding short glass fibre-reinforced plastics materials in no way improve the performance of tubes manufactured in that way. Actually, it is a well known fact that the fibres then tend to be directed in the direction of flow, and the reinforcing effect is then particularly sensitive in the longitudinal extent which is that subject to less pull, whilst in the radial extent the mechanical properties of the material are only slightly improved.
The need for the fibres to be directed mainly in the circumferential extent has often led to tube manufacturers using continuous fibres joined to the tube which has been extruded beforehand, using spool techniques, for example. The specification BE-684.102 is known, for example, wherein spool techniques are used to arrange a band of continuous fibres over a tube which has been extruded beforehand, and this band is covered over by a second plastics layer which is deposited by extrusion with a square head. The specification GB-1.052.884 is also known, wherein, still using a tube which has been extruded beforehand, a tubular braid of reinforcing fibres is arranged and covered over by a second plastics layer using an extrusion method with a square head. These techniques which are effective in reinforcing tubes are, however, awkward to carry out because they require two extruders, one of which is provided with a square head, a spool or a braiding machine and devices for holding the supporting internal tube during the braiding or spool operations, which can be quite complicated if proper adherence of the various plastics layers is to be achieved.
The U.S. Pat. No. 3,508,297 is also known, wherein a description is given of the direct passage of a tubular braid in an extrusion drawplate with a square head, and of the encasement of that braid within the plastics layer to form a reinforced tube. The device is delicate to use, mainly because a very special tool is needed to enable these braided tubes to pass through the drawplate whilst ensuring that it is sealed to molten plastics material and under pressure in the drawplate. Moreover, manufacturing devices, feed devices and braided tube support devices are needed upstream of the extruder.
This kind of process always requires extra equipment and expenditure with respect to the treatment of the reinforced continuous fibres and because of the need for the various layers deposited at different stages during the process to adhere together properly.
These problems do not exist if discontinuous fibres are used to reinforce the plastics material.
In fact, the materials reinforced by discontinuous fibres can be extruded without any major problems, and the patent GB-2.182.603 describes, for example, the manufacture of tubes using conventional extrusion methods followed by expansion of the tube by applying an internal pressure which places the tube flat against a conformator. The fact that the tube swells causes significant longitudinal and radial drawn out sections which can encourage orientation of some of the fibres in the circumferential direction, but this is accompanied by non-adhesion at the interface between the fibres and the polymer material at atmospheric pressure.
The U.S. Pat. No. 3,605,189 is also known which employs a conventional tube drawplate, but which is provided with a rotating mandrel.
Shearing in the radial extent is thus superposed over normal shearing in the direction of flow, and tends to direct at least some of the fibres in a circumferential direction which is perpendicular to the flow axis. In this case, the orientation depends mainly on the relative intensity of the shearing. With regard to the rotational speed of the mandrel, the orientation effect decreases when the shearing rate is increased in the longitudinal extent, for example by increasing the flow rate through the drawplate.
The U.S. Pat. No. 4,056,591 is also known wherein at the end of the drawplate a diverging zone is used, characterised by a ratio between the surface areas of the annular exit sections and of the annular entry sections which is greater than 2. In this device, the flowing substance is urged by shearing in the direction of flow along the walls of the drawplate and in the transverse extent in the divergence. If tubes manufactured in this way are studied, fibres will be seen which are directed perpendicularly to the direction of flow in the heart of the wall thickness, but longitudinally in the wall of the tube. The mechanical properties of the tube are inadequate.
An improvement to this process is described in the U.S. Pat. No. 4,883,622, and it consists in physically separating a zone of low thickness in the tube wall where there is a concentration of shearing. An uncharged polymer flows in that zone, whilst, flowing in the central zone is the charged polymer which extends transversely without shearing.
This kind of coextrusion device permits an increase in the number of fibres oriented perpendicularly to the direction of flow, and thus an increase in the resistance of the tube. However, if the resistance is viewed in consideration of the amount of material per unit of thickness, the process becomes less interesting since the uncharged peripheral layers of material play no part in the reinforcement. Moreover, the high cost of coextrusion is also to the detriment of this process.